In general, ink jet printing machines or printers include at least one printhead that ejects drops or jets of liquid ink onto a recording or image forming media. A phase change ink jet printer employs phase change inks that are in the solid phase at ambient temperature, but transition to a liquid phase at an elevated temperature. The molten ink can then be ejected onto a printing media by a printhead directly onto an image receiving substrate, or indirectly onto an intermediate imaging member before the image is transferred to an image receiving substrate. Once the ejected ink is on the image receiving substrate, the ink droplets quickly solidify to form an image.
In both the direct and offset printing architecture, images may be formed on a media sheet or a media web. In a web printer, a continuous supply of media, typically provided in a media roll, is mounted onto rollers that are driven by motors. A loose end of the media web is passed through a print zone opposite the print head or heads of the printer. Beyond the print zone, the media web is gripped and pulled by mechanical structures so a portion of the media web continuously moves through the print zone. Tension bars or rollers may be placed in the feed path of the moving web to remove slack from the web so it remains taut without breaking.
In a typical phase change ink direct printing system, melted phase change ink is ejected from jets in the print head directly onto the final receiving web. In phase change ink continuous-web printing, a high pressure roller nip, also referred to as a spreader, is used after the melted phase change ink is jetted onto the web to spread the ink on the web to achieve the desired print quality. The function of the spreader is to take what are essentially isolated droplets of ink on web and smear them out to make a continuous layer by pressure and/or heat so that spaces between adjacent drops are filled and image solids become more uniform. Other methods of spreading or fixing ink are also possible such as with heat or pressure alone.
Two difficulties faced in imaging devices, and in particular, imaging devices that utilize phase change ink to form images, are ink rub and offset during handling of the prints. As used herein, ink rub refers to the smearing or scuffing of the ink of an image on a receiving substrate, such as a sheet of paper. Ink offset refers to ink from an image formed on a surface or portion of a surface of a receiving substrate being transferred to another surface or another portion of the substrate. Ink rub and offset is particularly a concern for applications that require extensive handling such as the outside of envelopes or printed sheets inserted into envelopes.
To prevent ink rub and/or offset, some previously known systems utilize a protective coating, such as varnish, applied over the printed image on the substrate to prevent or minimize ink rub or offset of the printed image. For example, a varnisher places a protective coating over the entire image in order to prevent ink rub and/or offset from the resulting prints. In some previously known systems, overlaying clear ink may require coverages greater than 50% because the act of spreading the ink in the spreader brings all of the ink to a common level and if the overlay of clear ink does not completely cover the colored ink, some of the colored ink may end up at the surface where it is susceptible to rub and offset. While high coverage clear coatings applied over images formed on a substrate may be useful to prevent or minimize ink rub or offset of the images, the addition of the clear coating material adds to the expense of the print, and the increased expense due to the coating is commensurate with the amount of coating used per print.